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Re: pretensioned bolts

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Ted Partrick wrote: 
 
>There appears to be a misconception regarding the clamping stresses in
bolts 
in tension in connections.  It has been asserted that applied loads to the 
connection will relieve some of the stresses on bolts, loads up to the point 
where clamping stress equals stress due to applied loads.  My own 
understanding is that the only way to relieve tension stress in the bolt is
to 
remove the strain in it...in other words, let the bolt shorten to its
original 
length. < 
 
I have studied the "prestressed principle" before and agree in general with 
those that say that the stress in the bolt is not increased until the
applied 
load exceeds the pretension force.  However, I do think that a relatively 
small stress increase will occur under the applied load.  What happens is
that 
during pretensioning the bolt elongates and compresses the members that it
is 
clamping together.  As additional load is applied, the bolt strains slightly 
and the compression force in the clamped parts is reduced due to the
increased 
bolt length.  Thus the added applied load to the bolt is largely offset by
the 
reduction in pressure from the clamped members.  Since the bolt must
elongate 
slightly for this to occur, the stress in the bolt does increase.  However, 
the amount of strain to relieve the clamping pressure is far less than the 
amount of strain an applied load would cause in an unstressed bolt.  Thus
the 
stress increase in the bolt is relatively small compared to the applied
load.  
Once the applied load exceeds the pretension force, all of the clamping 
pressure has been relieved and any additional load causes full net strain in 
the bolt.  (I believe this concept applies equally to all prestressed
members, 
i.e. to prestressed concrete as well.)